The September 11 attacks that triggered the Afghanistan mission represented a "terrorist atrocity that still casts a shadow over our world", General Richards wrote.

He argued that while the threat from those behind the attacks had "diminished" following the death of Osama bin Laden - while many of his accomplices had been arrested or killed - they still posed a danger.

General Richards said Afghanistan had become a base for terrorism because the country had collapsed and its institutions disappeared.

"Over the past ten years we have been rebuilding them," he said. "The progress we have made is truly impressive."